1. Field of the Invention
This invention relates to a demodulation apparatus, broadcasting system and broadcast receiving apparatus and, more particularly, to a demodulation apparatus for demodulating modulated signals, a broadcasting system for communicating digital satellite broadcasts and a broadcast receiving apparatus for demodulating modulated signals for a digital satellite broadcast.
2. Description of the Related Art
Phase shift keying (PSK) is one of digital modulation demodulation techniques. PSK is a modulation technique in which a phase is changed as a parameter for a carrier, and is widely used in, for example, the satellite communication field.
FIG. 21 is a view showing the rough configuration of a conventional PSK demodulator. A PSK demodulator 400 comprises a local oscillator 401, multipliers 402a and 402b, a regenerating section 403, a frequency correction value outputting section 404, and a π/2 phase shifter 405.
The local oscillator 401 generates a sine wave the frequency and phase of which are the same as those of a carrier used for modulation on the sending side. The π/2 phase shifter 405 shifts by π/2 the phase of a signal sent from the local oscillator 401. The multiplier 402a calculates the product of an input signal and a signal sent from the local oscillator 401. The multiplier 402b calculates the product of the input signal and output from the π/2 phase shifter 405.
The regenerating section 403 causes the low-frequency components of signals output from the multipliers 402a and 402b to pass through it and makes an A/D conversion on them to generate digital signals corresponding to phase axes. Then the regenerating section 403 makes a frequency correction on the basis of frequency correction value Δf output from the frequency correction value outputting section 404. And then the regenerating section 403 performs timing recovery and carrier recovery and detects a unique word (synchronization code word). (Synchronization is detected when SYNC is “H.”) The detection of a unique word means that normal demodulation control has been performed.
In this case, the PSK demodulator 400 uses the local oscillator 401 being independent of the sending side, so it will be impossible to obtain exactly the same frequency and phase. Conventionally, the frequency correction value outputting section 404 outputs frequency correction values in turn until a unique word is detected. The regenerating section 403 performs frequency correction, timing recovery, and carrier recovery each time it receives a frequency correction value. Input signals have been demodulated by performing such feedback control.
However, when a modulation frequency for an input signal and the oscillation frequency of the local oscillator 401 differ significantly, demodulation by the conventional PSK demodulator 400 described above will take a long time and degrade signal quality.
At first, a frequency correction value is zero. After waiting for time TTMAX (s) which a timing recovering circuit needs to lock, the regenerating section 403 in the PSK demodulator 400 begins carrier recovery. Then the regenerating section 403 begins to detect a unique word after waiting for time TCMAX (s) which a carrier recovering circuit needs to lock. The locking of a unique word detecting circuit (it needs time TFMAX (s) to lock) means that a unique word is detected. As a result, SYNC will turn to “H.”
If SYNC is “L,” then the frequency correction value outputting section 404 updates frequency correction value Δf (Hz) and repeats the above operation. Usually a frequency correction value will be updated in turn to values (0→ΔA→−ΔA→2ΔA→−2ΔA→3ΔA→−3ΔA→ . . . , for example) set in advance on the basis of a symbol rate.
A cycle during which a frequency correction value is updated is given by TTMAX+TCMAX+TFMAX. Therefore, when a modulation frequency for an input signal and the oscillation frequency of the local oscillator 401 differ significantly, time TTMAX+TCMAX+TFMAX will be repeated many times until synchronization is detected. That is to say, in this case, demodulation takes a long time.